The present invention relates generally to the field of remote sensor systems and, more particularly, to an intrinsically safe, wireless, battery-operated sensor system that conserves battery power.
Industry and safety standards require that liquid storage tanks of the type located on petroleum tank farms be given periodic checks to determine the liquid level of each tank. Level indicators for such tanks typically comprise a tape and pulley apparatus with a float at one end of the tape within the tank and a mechanical level indicator at the other end.
This typical arrangement was improved on by Clarkson, U.S. Pat. No. 4,459,584, to include a transmitter for remote monitoring of liquid level, but the Clarkson system still required the use of the old tape and pulley system to monitor liquid level. This system suffered all of the common problems with the tape and pulley system in that such a system may mechanically jam, thereby disabling the movement of the float. Further, a large volume of volatile gases fills the conduit enclosing the tape and pulley, since this system is open to the tank. This feature alone presents a significant safety hazard. Further, the Clarkson system provides no means for calibration of the level detector, since it relies upon the old tape and pulley system.
We began the development of the present invention with a level monitoring and alarm system sold under the trademark Fillcheck(copyright). This system included a transmitter that was mounted in an off-the-shelf NEMA-7 explosion-proof enclosure so as to enable it to be used in the electrically hazardous environments associated with petroleum storage tanks, pipelines, oil refineries, petrochemical plants, and fuel terminals. This type of mounting solved the problems described with regard to Clarkson, but it brought about certain shortcomings, such as the attenuation of the level signal. Mounting the transmitter and its antenna inside the explosion-proof enclosure significantly reduced the system""s effective range. In this system, the explosion-proof transmitter had to be aimed toward its receiver or repeater for optimum performance. Further, that system was expensive, and the transmitter had to be equipped with a safety barrier so as to permit its connection to an external level switch, which added to system cost. That system was also heavy, in that explosion-proof enclosures are quite massive and add significantly to shipping and handling costs. Finally, explosion-proof enclosures are typically constructed of aluminum which is prone to pitting and corrosion, known as exfoliation, particularly when used in marine or coastal environments in which many refining and petrochemical facilities are located.
The recognition of these drawbacks of the Fillcheck(copyright) system led to the development of the system which is described and claimed in our U.S. patent application Ser. No. 09/541,353; filed Mar. 31, 2000; which will issue on May 8, 2001 as U.S. Pat. No. 6,229,448; the parent application of the present application. In that system, we disclosed an intrinsically safe transmitter which maybe used generally for sending digital signals derived from any analog sensor, including a sensor for temperature, pressure, flow rate, etc., as well as the tank level sensor for which the original system was specifically designed. The original impetus for the that system was over-fill protection for storage tanks in hazardous environments, and has proven to be successful.
In systems in which that system finds applications, sensors typically require some small amount of power to develop a sensed signal, which may then be manipulated into a signal for transmission. Although the amount of power drawn by such sensors is small, it is significant when one is dealing with remote, battery powered sensors which must operate without operator attention over a long period.
Thus, there remains a need for a sensor monitoring system which is intrinsically safe and operates only intermittently in order to conserve the precious resource of battery power. The present invention addresses this need in the art.
The system of this invention comprises four components: (1) a passive sensor; (2) a transmitter module which includes a microprocessor controller, or microcontroller; (3) a repeater (if required); and (4) a receiver. It provides easy installation, convenient calibration, and very low maintenance. It is specifically designed to provide safe and power-efficient sensor operation where external electrical power and/or alarm signal wiring to a supervised point are not available. Further, it is easily adapted to other systems which include a passive sensor providing an analog signal which is then transmitted by the system of this invention to a central monitor.
In a preferred embodiment of the invention, a passive sensor for a specific application is installed. Such a sensor may sense temperature, pressure, flow rate, vibration, torsion or other mechanical stress, level, or other parameters of interest which must be monitored periodically or otherwise on command. The sensor typically develops an analog or digital signal, which is coupled through an electrical conductor to the intrinsically safe transmitter. The sensor remains unpowered (dormant) until commanded by the microcontroller in the transmitter module to take a reading. The command from the microprocessor may be activated by a programmed clock signal within the microprocessor. When the transmitter module receives a sensed signal from the sensor, it transmits the sensed signal in digital form to a repeater (if necessary) and ultimately to the receiver at the central monitor.
The sensed signal from the sensor may also include an alarm function by including an alarm switch associated with the sensor. When the switch is activated, the transmitter broadcasts an alarm signal to the receiver. In the alarm mode of operation, a relay output at the receiver is either opened or closed thus activating the alarm or other device attached to it. The relay output may also connect to a control apparatus, such as a signal transmitter to provide for remote operation of such devices as valves.
The sensor and transmitter package are powered by the same or a secondary battery, such as for example a D cell size 3.6 V lithium thionyl chloride battery. During non-alarm conditions, a supervisory signal may be transmitted by the transmitter module under the control of the microcontroller every 30 seconds or at a selected interval. The supervisory signal contains the transmitter identification and battery condition. If the receiving system fails to hear from any of its transmitters, an inactive alarm is issued. Also, low battery alarms are issued before a transmitter""s battery dies.
Any type of passive sensing device that provides an analog or digital electrical output signal can be used with this embodiment of the present invention. For example, storage vessels may include a magnetostrictive level sensor, such as a liquid level sensing system from Ametek Patriot Sensors in Clawson, Mich. These devices can be constructed of stainless steel, brass, polypropylene, or other appropriate materials, and are available in a vertical configuration. Other passive sensing devices for other parameters to be sensed can also be used in accordance with this invention.
The system of the present invention provides reliable, low cost, wireless sensor monitoring capability which conserves battery power by activating either the transmitter module or the sensor only when needed. Many applications now require such remote monitoring. Thus, this system provides continuous, fully supervised protection against conditions which routinely require monitoring. It includes a battery powered transmitter, so it needs no external electrical power at the storage tank. The transmitter is intrinsically safe (Class I Division 1 Groups CandD), and requires no FCC licensing. It has low installation and maintenance costs, and the alarms are provided with fail-safe aspects for increased reliability.